text: #define DEBUG 1
  /**
   * File    : B.cpp
   * Author  : Kazune Takahashi
   * Created : 2/7/2020, 2:11:02 PM
   * Powered by Visual Studio Code
   */
  #include <algorithm>
  #include <bitset>
  #include <cassert>
  #include <cctype>
  #include <chrono>
  #include <cmath>
  #include <complex>
  #include <cstdint>
  #include <cstdio>
  #include <cstdlib>
  #include <functional>
  #include <iomanip>
  #include <iostream>
  #include <map>
  #include <queue>
  #include <random>
  #include <set>
  #include <stack>
  #include <string>
  #include <tuple>
  #include <unordered_map>
  #include <unordered_set>
  #include <vector>
  // ----- boost -----
  #include <boost/rational.hpp>
  #include <boost/multiprecision/cpp_int.hpp>
  // ----- using directives and manipulations -----
  using namespace std;
  using boost::rational;
  using boost::multiprecision::cpp_int;
  using ll = long long;
  template <typename T>
  using max_heap = priority_queue<T>;
  template <typename T>
  using min_heap = priority_queue<T, vector<T>, greater<T>>;
  // ----- constexpr for Mint and Combination -----
  constexpr ll MOD{1000000007LL};
  // constexpr ll MOD{998244353LL}; // be careful
  constexpr ll MAX_SIZE{3000010LL};
  // constexpr ll MAX_SIZE{30000010LL}; // if 10^7 is needed
  // ----- ch_max and ch_min -----
  template <typename T>
  void ch_max(T &left, T right)
  {
    if (left < right)
    {
      left = right;
    }
  }
  template <typename T>
  void ch_min(T &left, T right)
  {
    if (left > right)
    {
      left = right;
    }
  }
  // ----- Mint -----
  template <ll MOD = MOD>
  class Mint
  {
  public:
    ll x;
    Mint() : x{0LL} {}
    Mint(ll x) : x{(x % MOD + MOD) % MOD} {}
    Mint operator-() const { return x ? MOD - x : 0; }
    Mint &operator+=(const Mint &a)
    {
      if ((x += a.x) >= MOD)
      {
        x -= MOD;
      }
      return *this;
    }
    Mint &operator-=(const Mint &a) { return *this += -a; }
    Mint &operator++() { return *this += 1; }
    Mint &operator++(int)
    {
      Mint tmp{*this};
      ++*this;
      return tmp;
    }
    Mint &operator--() { return *this -= 1; }
    Mint &operator--(int)
    {
      Mint tmp{*this};
      --*this;
      return tmp;
    }
    Mint &operator*=(const Mint &a)
    {
      (x *= a.x) %= MOD;
      return *this;
    }
    Mint &operator/=(const Mint &a)
    {
      Mint b{a};
      return *this *= b.power(MOD - 2);
    }
    Mint operator+(const Mint &a) const { return Mint(*this) += a; }
    Mint operator-(const Mint &a) const { return Mint(*this) -= a; }
    Mint operator*(const Mint &a) const { return Mint(*this) *= a; }
    Mint operator/(const Mint &a) const { return Mint(*this) /= a; }
    bool operator<(const Mint &a) const { return x < a.x; }
    bool operator<=(const Mint &a) const { return x <= a.x; }
    bool operator>(const Mint &a) const { return x > a.x; }
    bool operator>=(const Mint &a) const { return x >= a.x; }
    bool operator==(const Mint &a) const { return x == a.x; }
    bool operator!=(const Mint &a) const { return !(*this == a); }
    const Mint power(ll N)
    {
      if (N == 0)
      {
        return 1;
      }
      else if (N % 2 == 1)
      {
        return *this * power(N - 1);
      }
      else
      {
        Mint half = power(N / 2);
        return half * half;
      }
    }
  };
  template <ll MOD>
  Mint<MOD> operator+(ll lhs, const Mint<MOD> &rhs)
  {
    return rhs + lhs;
  }
  template <ll MOD>
  Mint<MOD> operator-(ll lhs, const Mint<MOD> &rhs)
  {
    return -rhs + lhs;
  }
  template <ll MOD>
  Mint<MOD> operator*(ll lhs, const Mint<MOD> &rhs)
  {
    return rhs * lhs;
  }
  template <ll MOD>
  Mint<MOD> operator/(ll lhs, const Mint<MOD> &rhs)
  {
    return Mint<MOD>{lhs} / rhs;
  }
  template <ll MOD>
  istream &operator>>(istream &stream, Mint<MOD> &a)
  {
    return stream >> a.x;
  }
  template <ll MOD>
  ostream &operator<<(ostream &stream, const Mint<MOD> &a)
  {
    return stream << a.x;
  }
  // ----- Combination -----
  template <ll MOD = MOD, ll MAX_SIZE = MAX_SIZE>
  class Combination
  {
  public:
    vector<Mint<MOD>> inv, fact, factinv;
    Combination() : inv(MAX_SIZE), fact(MAX_SIZE), factinv(MAX_SIZE)
    {
      inv[1] = 1;
      for (auto i = 2LL; i < MAX_SIZE; i++)
      {
        inv[i] = (-inv[MOD % i]) * (MOD / i);
      }
      fact[0] = factinv[0] = 1;
      for (auto i = 1LL; i < MAX_SIZE; i++)
      {
        fact[i] = Mint<MOD>(i) * fact[i - 1];
        factinv[i] = inv[i] * factinv[i - 1];
      }
    }
    Mint<MOD> operator()(int n, int k)
    {
      if (n >= 0 && k >= 0 && n - k >= 0)
      {
        return fact[n] * factinv[k] * factinv[n - k];
      }
      return 0;
    }
    Mint<MOD> catalan(int x, int y)
    {
      return (*this)(x + y, y) - (*this)(x + y, y - 1);
    }
  };
  // ----- for C++14 -----
  using mint = Mint<MOD>;
  using combination = Combination<MOD, MAX_SIZE>;
  template <typename T>
  T gcd(T x, T y) { return y ? gcd(y, x % y) : x; }
  template <typename T>
  T lcm(T x, T y) { return x / gcd(x, y) * y; }
  // ----- for C++17 -----
  template <typename T>
  int popcount(T x) // C++20
  {
    int ans{0};
    while (x != 0)
    {
      ans += x & 1;
      x >>= 1;
    }
    return ans;
  }
  // ----- frequently used constexpr -----
  // constexpr double epsilon{1e-10};
  // constexpr ll infty{1000000000000000LL};
  // constexpr int dx[4] = {1, 0, -1, 0};
  // constexpr int dy[4] = {0, 1, 0, -1};
  // ----- Yes() and No() -----
  void Yes()
  {
    cout << "Yes" << endl;
    exit(0);
  }
  void No()
  {
    cout << "No" << endl;
    exit(0);
  }
  // ----- main() -----
  
  int main()
  {
    int N;
    cin >> N;
    vector<int> A(N);
    for (auto i = 0; i < N; ++i)
    {
      cin >> A[i];
    }
    int S{accumulate(A.begin(), A.end(), 0)};
    int T{0};
    for (auto i = 0; i < N; ++i)
    {
      T += A[i] * A[i];
    }
    cout << (S * S - T) / 2 << endl;
  }
  